Journal of Neurophysiology, Vol 66, Issue 5 1471-1482, Copyright © 1991 by APS

ARTICLES

Effect of metabolic inhibition on the excitability of isolated hippocampal CA1 neurons: developmental aspects

T. R. Cummins, D. F. Donnelly and G. G. Haddad
Department of Pediatrics, Yale University School of Medicine, New Haven, CT 06510.

1. The effects of brief exposures to hypoxia on the membrane currents of isolated hippocampal CA1 neurons were studied with the use of the whole-cell variation of the patch-clamp technique. Neurons were acutely dissociated from immature (day 2-7) and mature (day 21-43) rats. 2. In the current-clamp mode, Na-cyanide (CN) hyperpolarized both mature and immature neurons. In the voltage-clamp mode, CN decreased the magnitude of the hyperpolarizing holding current in both age groups. 3. CN did not have a consistent effect on the voltage-dependent calcium and potassium currents of immature and mature CA1 neurons but decreased the voltage-dependent inward current of neurons at both ages. This effect was age dependent: the inward current of immature neurons decreased by only 10%, but that of mature neurons decreased by approximately 40%. 4. The decrease in the magnitude of the hyperpolarizing holding current and the depression of the voltage-dependent inward current of mature neurons were observed during brief exposure to N2 (PO2 = 0), indicating that the electroresponses observed with CN were the result of blocking oxidative respiration. 5. The hypoxia-sensitive inward current was blocked by tetrodotoxin (TTX) but was not blocked by cadmium or cesium + tetraethylammonium (TEA). Therefore this current was identified as the voltage-dependent, fast-inactivating sodium current (INa). 6. The isolated sodium current was studied with the use of cadmium to block calcium and TEA + cesium to block potassium currents. In mature neurons, CN left-shifted the steady-state inactivation curve for INa and slowed the deactivation kinetics of INa. CN caused little or no change in INa activation, fast inactivation, recovery from inactivation, or current-voltage (I-V) relationship. 7. We conclude that brief exposures to CN and hypoxia alter the intrinsic excitability of CA1 neurons by at least two mechanisms: 1) alterations in leakage currents and 2) alterations in the fast Na+ conductance that are maturationally dependent. We propose that the alterations in the Na+ conductance may play an adaptive role by reducing O2 demands and thus possibly delaying neuronal injury.

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